Sites of Abscisic Acid Synthesis and Metabolism in Ricinus communis L.

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Abstract
The sites of abscisic acid (ABA) synthesis and metabolism in R. communis L. were investigated by analyzing the levels of ABA and its 2 metabolites phaseic acid (PA) and dihydrophaseic acid (DPA) in the shoot tips, mature leaves and phloem sap of stressed and nonstressed plants. Water stress increased the concentration of ABA, PA and DPA in phloem exudate and increased the levels of all 3 compounds in mature leaves and in shoot tips. The latter had a very high DPA content (18.7 .mu.g/g fresh wt) even in plants not subjected to water stress. When young and mature leaves were excised and allowed to wilt, the level of ABA increased in both, demonstrating that leaves at an early stage of development have the capacity to produce ABA. Apparently, in mature leaves of nonstressed Ricinus plants, ABA is synthesized and metabolized and ABA itself and its metabolites, are translocated in the phloem to the shoot tips (sinks). Since DPA, but not ABA, accumulates in the shoot tips, it follows that ABA is metabolized rapidly in the apical region. To what extent ABA present in young leaves of nonstressed plants is the consequence of synthesis in situ and of import from older leaves remains to be determined.